Specific gravity analyzer for control of an alkylation process

ABSTRACT

The acid strength of a process stream associated with an alkylation process using a mineral acid catalyst, such as sulfuric acid, can be conveniently and continuously measured by determining the specific gravity of a sample of acid taken from the main process stream. The sample is pumped under positive and fixed flow rates into and from an analysis chamber by means of first and second pumping means, connected, respectively, to the inlet and outlet of the chamber. Prior to determining the gravity of the acid within the chamber, dissolved volatile hydrocarbons and high molecular weight, normally liquid, hydrocarbons are removed from the sample by stripping and settling chambers within the analysis chamber, the high molecular weight hydrocarbons flowing from the chamber by means of an overflow tube positioned in the settling chamber. The weight percent of the carbonaceous matter removed from the sample including the high molecular weight hydrocarbons is controlled by maintaining said second pumping means at a pumping rate less than that of said first pumping means.

United States Patent ilrandel [451 Apr. 4, 1972 [54] SPECIFIC GRAVITYANALYZER FOR Prima y Examiner-Morris k CONTROL OF AN ALKYLATIONAssistant Examiner-Elliott A. Katz PROCESS Att0meyA. L. Snow, F. E.Johnston, R. L. Freeland. Jr. and

Harold D. Messner [72] inventor: Albert John Brande], Long Beach, Calif.[73] Assignee: Chevron Reasearch Company, San Fran- [57] ABSTRACT c1500Cahf' The acid strength of a process stream associated with an al- 22Filed; 20, 1969 kylation process using a mineral acid catalyst, such assulfuric acid, can be conveniently and continuously measured by [21] PPV-F 7, A o M determining the specific gravity of a sample of acid takenfrom I the main process stream. The sample is pumped under positiveRelated US. Application Data and fixed flow rates into and from ananalysis chamber by DIVISIOII of n- 19, 1968, Pat. means of firstandsecond pumping means, connected, respec- H QLV1 L3JZZ V, a a ltively, to the inlet and outlet of the chamber. Prior to deter- [52] U 5Cl 23/230 R 23/230 A 73/32 mining the gravity of the acid within thechamber, dissolved R 23/253 A 26968359 volatile hydrocarbons and highmolecular weight, normally [51] mt CL "501 9/18 G01 9/36 C076 3/14liquid, hydrocarbons are removed from the sample by [58] Field of 23/23673/35 445447 stripping and settling chambers within the analysischamber, 260/683 683 the high molecular weight hydrocarbons flowing fromthe chamber by means of an overflow tube positioned in the settlingchamber. The weight percent of the carbonaceous matter [56] Referencescued removed from the sample including the high molecular weight UNITEDSTATES PATENTS hydrocarbons is controlled by maintaining said secondpump- 3 293 320 12/1966 M I t 1 260/683 59 ing means at a pumping rateless than that of said first pumpagear e a in means 2,592,063 4/1952Persyn, Jr. .260/683.59 X g 2,850,552 9/1958 Ogle ..260/683.43 7 Claims,3 Drawing Figures VENT 3 ea 1 II 'O so I 59 9 A II 40 5/0 3 76 4 47 a ll59 a l if $62 Ii 79 7/ T I I 5.: i

HQ 4 =01 i ii H H l All To ACID BATH Ii rzeo l II I a a n \r L H I a H Has 77b H H H II I1 I! H H H I ACID .1; 43 A o RAIN 45 \WATER m1 DRAIN ACSUPPLY SPECIFIC GRAVITY ANALYZER FOR CONTROL OF AN ALKYLATION PROCESSThe present invention is concerned with a method and apparatus for thedetermination of the concentration of solutions. In particular, thepresent invention is directed to the determination and control of acidstrength in chemical processes dependent on acid catalysis by means ofmeasuring the specific gravity of the solution. More particularly, theinvention is directed to the determination and control of acid strengthin a sulfuric acid catalyzed alkylation reaction in which isoparaffinhydrocarbons and olefin hydrocarbons are reacted in contact with asulfuric acid catalyst. Still more particularly, this invention relatesto a method and means for measuring the concentration of spent sulfuricacid by continuously measuring the specific gravity of a sample of suchacid.

The present invention may be briefly described as a method formaintaining catalyst activity in the aforementioned sulfuric acidcatalyzed alkylation reaction in which the sulfuric acid gradually losesits strength for the alkylation reaction thereby requiring thewithdrawal of a portion of the sulfuric acid in the system and thereplacement of the withdrawn portion with sulfuric acid of apredetermined alkylation strength.

The method of alkylating isoparaffins and olefins in the presence of aconcentrated sulfuric acid catalyst is well known in the petroleumrefining art. Typically, in this process, isobutane and butylenes arealkylated using a concentrated sulfuric acid catalyst which may rangeupward in strength from about 85 weight percent H 50 The acid strengthtends to decrease during the operation of the process due to dilutionwith carbonaceous matter such as sulfonated hydrocarbons, i.e.,catalyst-hydrocarbon complexes and esters, and with water. As the acidis diluted, its activity decreases. In the typical instance, thealkylation reaction is carried out in a series of mixing zones withoutthe addition of acid to the materials passed through the zones. Forsatisfactory operation the acid strength in the last reactor must bemaintained above a certain minimum concentration. When the acid strengthdrops below an established minimum, the acid is discarded. It is usualto recycle a portion of the spent alkylation acid inasmuch as ispossible to reduce the operating costs of the process, since the cost ofacid used is important. Thus, it is desirable to minimize the amount ofspent acid discarded because of the drop in acid strength. However, thedetermination of the acid strength is time consuming and expensive whencarried out in the laboratory, and laboratory methods do not providesufficient rapidity for close operational control ofthe process.

Since almost all commercial processes which utilize an acid catalystinvolve continuous recycling of the acid catalyst through the reaction,it is of great importance to be able to continuously and accuratelydetermine the acid strength. When the acid strength falls below theminimum amount necessary, additional fresh acid can be added. However,if the analytical technique is not accurate, then either too much acidmay be added, resulting in a wasting of this expensive material, or elsetoo little may be added, resulting in poor product yields.

Heretofore, the acidity of the alkylation acid could be determinedspectrophotometrically, but this technique suffers from severaldisadvantages:

1. it requires the use of relatively expensive spectrophotometers;

2. it requires a continuous addition ofan indicator compound such asalizarin blue in known amounts to the sample. Accordingly, a complicatedand expensive control and metering system for the indicator compound isneeded. 1

In a further prior art method, the acidity function of an alkylationacid could be determined by continuously measuring the specific gravityof a sample of acid by means of a hydrometer after removal ofcarbonaceous matter from the sample. Experience has shown that inflowing the sample to the hydrometer clogging often occurs due to thedeposit of carbonaceous matter in pressure-reducing elements of thesystem such as orifice restrictors.

It is therefore an object of the present invention to provide a usefuland novel method and apparatus for the accurate and continuousdetermination of the specific gravity of reaction process streamsutilizing only relatively simple and inexpensive instruments which willnot require the addition of indicators or other reagents and which donot clog even though the sample contains carbonaceous matter. It is afurther object to provide such a method and apparatus which can beadapted to provide automatic control over a continuous acid catalyzedchemical process so as to maintain maximum product yields with minimalacid losses.

It has been found that the specific gravity and especially changes inthe specific gravity of a recycle process acid stream associated with analkylation process using a mineral acid catalyst, such as sulfuric acid,can be conveniently and continuously measured to determine theconcentration of the aforementioned recycle stream. A sample of thestream is pumped under positive and fixed flow rates into and from ananalysis chamber by means of first and second pumping means, connected,respectively, to the inlet and outlet of the chamber. Prior todetermining the gravity of the acid within the chamber, dissolvedvolatile hydrocarbons and high molecular weight, normally liquidhydrocarbons are removed from the sample by stripping and settlingchambers within the analysis chamber, the high molecular weighthydrocarbons flowing from the chamber by means of an overflow tubepositioned in the settling chamber. The weight percent of thecarbonaceous matter removed from the sample including the high molecularweight hydrocarbons is controlled by maintaining said second pumpingmeans at a pumping rate less than that of said first pumping means.

In a specific embodiment of the invention, a continuous sample of therecycle process stream is pumped, by means of an input acid pump, to thestripping chamber. To maintain a pumping head for the pump, apressure-sensitive relief valve may be fitted between the discharge sideof the pump and the stripping chamber operated to open on the dischargestroke of the pump when a working pressure above the process pressure isachieved. The pump and sequentially operated valve provide a constantrate of flow to the separation chamber without the use of restrictors.In the stripping chamber the sample is heated to the desired temperaturesay, F by means of a temperature-controlled hot water bath. While withinthe bath, the sample flows through the stripping chamber whichpreferably includes a substantially horizontal extending planar baseadapted to convey the input sample in open channel heat transfer contactwith the bath so as to strip the volatile hydrocarbons from the sample.The volatile hydrocarbons are vented to the atmosphere from the top ofthe stripping chamber as vapors. Bafi'les on the planar base of thestripping chamber provide a sinuous flow pattern to the sample. Withinthe settling chamber located at one end of the base in fluidcommunication with the stripping chamber, the high molecular weighthydrocarbons including the alkylate are skimmed off by an overflow tube.An outlet near the bottom of the settling chamber is connected to thesecond fluid pumping means for removing the stripped acid sample fromthe chamber after the sample has passed through both the stripping andthe settling chambers. Preferably, the overflow outlet tube within thestripper chamber is adapted to be positioned above the plane of the baseof the stripping chamber, the amount of the projection therebyregulating the thickness of the sample stream within the strippingchamber to a predetermined relatively small magnitude whereby uniformheat contact between the stripping and the heating chamber readilyoccurs. Prior to removal of the stripped acid from the settling chamber,the specific gravity of the acid is determined by means of a displacemeter of constant volume in which the buoyant force is measured by meansof a strain gage-type transducer so as to indicate the specific gravityof the acid sample.

By suitably controlling the pumping rates of the input and outputpumping means, a ratio of fluids removed by the overflow tube and by thesecond fluid pumping means can be controlled which assures thatsubstantially all volatile and high molecular weight hydrocarbons of thesample are removed prior to the gravity determinations.

If desired, the process from which the sampie is taken may be controlledby utilizing the specific gravity determinations to control valves whichwithdraw spent acid from the system and admit new acid of correctstrength to the system to maintain the strength of the acid at a propersetpoint level. When the acid strength and therefore the specificgravity falls below a preselected level, the control equipment connectedto the valves'may be automatically actuated in response to the deviationof the measured specific gravity until the optimum level within thesystem is again reached. It is thus possible to utilize such a system tomaintain the acid strength equilibrating about the optimum level therebyachieving a high process efficiency.

The invention may be used to control the catalyst activity of a singleor a plurality of alkylation reactors using common isoparaffin andolefin reactants in the presence of a mineral acid catalyst such assulfuric acid, hydrofluoric acid, etc.

The present invention will be further illustrated by reference to thedrawings in which:

FIG. 1 is an illustrative flow diagram of a typical hydrocarbon sulfuricacid alkylation process in which the system of the present invention canbe utilized;

FIG. 2 is an elevational view partially cut away of the apparatus ofFIG. 1 for processing a sample of the recycle stream in accordance withthe present invention todetermine the specific gravity of the sample;

FIG.'3 is a plan view, partially cut away, of the apparatus of FIG. 2illustrating in detail the flow of a sample within the stripping andsettling chambers of the apparatus of FIG. 2.

Referring to FIG. 1, there is illustrated diagrammatically a specificform of alkylation process conventionally used in the hydrofining artfor the purpose of illustrating the use of the present invention. Inoperation, a large stream of hydrocarbons undergoes alkylation in thepresence of an acid catalyst such as sulfuric acid. The hydrocarbons andcatalysts are continuously circulated by pump 10, through cooler 11,system line 12, reactor 13 and return line 14. Thorough mixing of thehydrocarbons with the acid is maintained by the mixing action of thepump and also by baffling (not shown) within the reactor. A side streamof the mixture is continuously withdrawn from a convenient point of thesystem, say through line 17, to settler 18 where the acid is permittedto settle from the-alkylated and nonreacted hydrocarbons. The settledacid is returned to the system through line 19 and the hydrocarbons arewithdrawn through line 20 for neutralization and fractionation intoalkylate products as well as recycle products. Makeup hydrocarbonsincluding the recycle products from fractionation are introduced to thesystem at a convenient point, as near the intake of punip 10, asillustrated by line 22.

The strength of the" acid circulated in the system gradually decreasesuntil a point is reached where its strength is uneconomically low forthe production of good quality alkylate. If not corrected, the acidstrength may become so low that various undesirable side reactionsoccur. To-maintain the acid strength at an economic optimum, spent acidis withdrawn from time to time through valve 24 and strong makeup acidat the alkylation concentration is introduced through valve 25. If thestrength of the acid within the reactor is not accurately andcontinuously determinable, determinations as to the amount of spent acidto be withdrawn and the amount of alkylation acid to be added to thesystem cannot be made. In withdrawing a sample of the acid from thesystem for such determination, several factors must be consideredincluding reducing, to a minimum, the time lag between withdrawal of thesample of the acid and the determination of the strength of the acid.Further difficulties can occur due to the pressure and temperatureprevailing within the system. For example, upon release of pressure, asample withdrawn immediately becomes foamy and must be allowed tosettle. Further, volatile and high molecular hydrocarbons must beremoved from the sample if the gravity determinations are to have anyreproducible significance. Still further difficulties occur in the useof orifice restrictors in withdrawing a sample of the acid from thesystem. Not only is the acid difficult to handle, the carbonaceousmatter within the withdrawn sample frequently clogs the restrictorsrequiring the installation of an attendant control system for cyclingsteam through the sampling system.

In accordance with the present invention, the spent alkylation acid issampled by means of sampling line 31 and flows at a constant ratethrough valve 32 to first stage acid pump 33 and pressure-sensitiverelief valve 34. The acid pump 33 discharges at a substantially constantflow rate through the valve 34; typically the flow rate is low so as tobe compatible with the size of the sampling system, say about 1.5gallons per hour.

The function of the pump 33 and relief valve 34'is to dissipate systempressure in passing the sample from process line 19 where, say, apressure of l20 p.s'.i.g. exists, to analysis chamber 35 where, say, apressure of about zero psig exists while simultaneously maintaining thesample at a substantially constant flow rate. Relief valve 34 is shownconnected to the discharge side of pump 33 and is adapted to be biasedto a full open position when a preselected discharge pressure above thesystem pressure occurs in line 36. The full open position of the valveis adapted to be large enough so that carbonaceous matter in the sampledoes not accumulate at the valve seat. The discharge pressure of theacid pump 33 periodically rises above the setpoint level of the valve 34to allow flow of the sample through the valve 34. The bypass line 37which includes valve 38 is placed in parallel with the relief valve 34.Valve 38 is normally closed but can be open to facilitate additionalflow from the acid pump 33.

As an alternative modification, the system pressure can also bedissipated at the suction side of acid pump 33. For example, apressure-sensitive valve may be positioned on the suction side of theacid pump 33 and be adapted to incrementally open to maintain a givenpressure drop between the valve and a pressure-sensing means'on thedischarge side of the pump. Accordingly, as carbonaceous matteraccumulates within the valve, the valve plug is incrementally releasedto thereby maintain a given flow rate through the acid pump.

The discharge of pump 33throu'gh' valve 34 is then passed by way of line39 vto the inlet of analysis chamber 35. As shown, chamber 35 ispositioned within a constant temperature bath 40 and, includes strippingchamber 41 and settling chamber 42. Bath 40 is employed to maintain thesample within the chamber 35 at a substantially constant temperature,say about F. For this purpose, steam-is passed into'the bath by way ofline 43 through temperature responsive valve 44. The bath is alsopreferably agitated by'air introduced to the chamber 35 through line 45.The sample is first preheated by exchanger 46 in contact with bath 40and is then passed into stripping chamber 41.

Stripping chamber 41 includes a planar base 47, .upright .walls 48attached to the base 47 and baffle plates 49. As the sample circulatesthrough the stripping chamber, the volatile hydrocarbons are heated to agaseous phase. The vapors exit through vent tube 50. Discharge of thestripping chamber is by way of the settling chamber 42. Settling chamber42 is provided with an overflow tube 51 and a receiving cavity 52. Asshown, cavity 52 is positioned below the skimming level established byinlet 51A of the overflow tube 51. Within the settling chamber 42, thehigh molecular weight hydrocarbons in the sample are skimmed off by theoverflow tube 51 and are passed from the settling chamber by way of line53. The stripped sample passes through the cavity 52 toward outlet 55near the floor of the settling chamber. Within cavity 52 is the displacemeter 56 weighted, i.e. with mercury, to a higher density than themaximum sample density to be measured,,and connected by a wire line57-to a buoyancy force reading transducer 58. Transducer 58 converts themagnitude of the buoyancy force acting on the displace meter 56 to anelectrical signal indicating a specific gravity of the stripped sampleThe gravity-indicating signal is then recorded by recorder 59 to providea record indicating the concentration of the alkylation acid.

Second stage acid pump 60 attaches to outlet 55 of cavity 52 for pumpingthe stripped sample from the settling chamber via line 61. The dischargeside of the acid pump 60 attaches via line 62 to a hydrometer pot 63.Specific gravity determinations of the stripped sample are made by ahydrometer in hydrometer pot 63 from time to time to calibratetransducer 58. Thus by monitoring the record of recorder 59, theoperator may be readily advised of the strength of the acid in thealkylation system. Desired corrections in the alkylation process can bemade manually or automatically by adding alkylation acid and withdrawingspent acid from the process.

To automatically control the addition of alkylation acid, recorder 59may be provided with mechanical linkages 64 which control spent acidvalve 24. As deviations from a setpoint level within therecorder-controller occur, valve 24 is adjusted to withdraw spent acidfrom the process. To control the admission of strong alkylation acid, tocompensate for the removal of the spent acid, settler 18 may be providedwith a level-indicating controller 65 for operating valve 25.Accordingly, as the acid hydrocarbon interface level within settler 18deviates from the preselected setpoint level, the valve 25 is actuatedto admit alkylation acid to the process. When an optimum concentrationof alkylation acid is achieved within the alkylation process, valves 24and 25 are biased to a closed position. Alternatively, therecorder-controller which includes recorder 59 and linkages 64 mayoperate spent acid valve 25 instead of valve 24. Accordingly,level-indicating controller 65 of settler 18 operates spent acid valve24.

FIGS. 2 and 3 illustrate the use of the gravity-indicating apparatus ofthe present invention in marked detail. In FIG. 2, a frame 70 is seen tobe fitted with lateral channel bars 71 at the upper end of the frame towhich analysis chamber 35 is attached. At the lower periphery, channelbars 72 and support base 73 provide support for acid pumps 33 and 60.Hydrometer pot 63 is positioned at one side of the frame 70 whilerecorder 59 is positioned at an opposite side, as shown, or remotely, asin a control room.

In flowing an acid sample under pressure to the apparatus of the presentinvention, the first stage acid pump 33 periodically creates a dischargepressure at discharge side 74 above the setpoint bias of relief valve 34as previously mentioned. Accordingly, acid pump 33 is preferably of thepiston or diaphragm type and may be operated as shown by a motordrivensystem also used to drive second stage acid pump 60. In such a drivesystem, a single motor 76 drives both pumps 33 and 60 through shafts 76Aand 76B to separately driven gear boxes 77A and 77B within each pump.Duplex version, Model No. SL-3, Lapp Insulator Company, Inc., Le Roy,New York, has been found to be a satisfactory, commercially available,tandem pump.

Bath 40 of analysis chamber 35 is enclosed within a support housing 78.Water comprising the bath is placed in heat transfer contact with thepre-heat exchanger 46 as well as stripping chamber 41 and settlingchamber 42. The temperature of the bath is controlled by steam enteringthe analysis chamber through the inlet line 43. The flow rate of thesteam is controlled by temperature-sensitive diaphragm valve 44controlled by temperature-measuring element 79. An exterior indicatingdial 80 provides visual indication of the temperature of the bath.

The temperature within the analysis chamber 35 must be maintained at auniform temperature during operations. If the temperature remainsconstant, substantially the same amount of hydrocarbons are removed fromthe acid sample within the analysis chamber. A single bath 40 for bothheating the sample to remove volatile hydrocarbons as well asmaintaining the settling chamber at a constant temperature as a specificgravity characteristic of the acid is measured, has been found to be animportant advantage of the present invention.

However, the temperature selected must be high enough to separatesubstantially all of the volatile hydrocarbons within the strippingchamber without too much troublesome foam. It should not be so high,however, as to substantially decompose the high molecular weighthydrocarbons, normally in the liquid phase, removed by skimming withinthe settling chamber. A selected temperature of about F. has proven tobe satisfactory although a range somewhere between 90 F. should prove tobe satisfactory in the average installatron.

As shown best in FIG. 2, the stripping chamber 41 is positioned at anelevated location above the floor of the bath housing 78. It is attachedin the elevated position by welding its planar base 47 and side walls 48to adjacent portions of the bath housing 78 in the settling chamber 42,as shown. When the level of the heater within bath housing 78 isadjusted to a level so that contact is made both with the side walls 48and the base 47 of the stripping chamber, as shown, heating of thesample occurs at the sides as well as the base of the stripping chamber.

FIG. 3 illustrates the flow pattern of the acid sample within thestripping chamber 41 in more detail. As shown, the stripping chamber 41is provided with a series of upright plates 49 attached to base 47 andthe side walls 48. After the acid sample has passed into the strippingchamber by way of inlet 76 and helically wound pre-heating tube 46, theplates 49 direct the sample in the sinuous flow pattern as shown. Theincremental time period that the sample contacts bath 40 is thusincreased. Accordingly, substantially all of the volatile hydrocarbonsare stripped from the sample. The vapors vent to atmosphere by way ofvent tube 50.

As shown in FIG. 2, settling chamber 42 includes side walls 81 attachedto the bath housing 78 in fluid communication with the stripping chamber48 through aperture 82 in the base 47. The planar base 83 is attached tothe floor of the bath housing. Base 83 is provided with openings bywhich overflow tube 51 and outlet 55 are rigidly positioned so as tocontact the fluid within the settling chamber 42. After the sample hasentered the settling chamber, the high molecular weight hydrocarbons arestripped from the sample at a skimming level established at the inlet51A of the overflow tube 51. The skimmed hydrocarbons are disposed todrain via line 53 at- .tached to the overflow tube 51. The stripped acidsample is then passed through cavity 52 below the skimming levelestablished by the overflow tube and then is conveyed from the settlingchamber by way of outlet 55 to the second stage pump 60.

In addition to establishing a skimming level for removing the highmolecular weight hydrocarbons, the overflow tube 51 also performsanother important function in the operation of the gravity-indicatingapparatus of the present invention.

As shown in FIG. 2, the skimming level established by overflow tube 51only slightly exceeds the plane of base 47 of the stripping chamber 41,say by the incremental distance d as shown. The depth of the samplewithin the stripping chamber is, of course, equal to the incrementaldistance d since the pressure in the stripping and settling chambers isthe same. Accordingly, the depth of the sample can be easily reduced toa relatively small magnitude, say one-eighth to one-half inch withoutintroducing inaccuracies in the gravity measurements later performed inthe settling chamber. As the removal of the hydrocarbons from the acidsample is enhanced by the apparatus of the present invention,measurements of the specific gravity characteristic of the strippedsample within cavity 52 of the settling chamber 42 result in a true andeasily reproducible indication of the specific gravity of the acid.

The pumping rates of the acid pumps 33 and 60 closely control thegravity-indicating apparatus of the present invention so that thepumping rate of the input, or first stage, acid pump 33 is alwaysgreater than that of the output, or second stage, acid pump 60.Accordingly, not only is the original acid sample pumped under positiveflow conditions into the analysis chamber 35 by means of acid pump 33,thereby reducing clogging in the measuring system to a minimum, but thestripped sample is also seen to be pumped under positive flow conditionsfrom the settling chamber 42 by means of second stage acid pump 60.

Furthermore, it can be seen that the differential pumping rateestablished by the first stage and second stage pumps can easily bevaried to values that assure that all of the hydrocarbons are removedfrom the acid sample prior to measuring the specific gravitycharacteristic of the stripped sample within cavity 52 of the settlingchamber 42. The skimming level of the fluid within the analysis chamberis preferably constructed to remain fixed during operations.Accordingly, the percentage by weight of the hydrocarbons removed fromthe acid sample is a direct function of the differential pumping ratesof the first stage and second stage acid pumps. Stated mathematically,for the minimum case: Weight Percentage hydrocarbons k) hwut where PR isthe differential flow rate of the first stage and second stage acidpumps and R is the flow rate of the first stage acid pump. in actualpractice, the pumping differential is always adjusted so that the ratiodefined above is always greater than the expected weight percentage ofhydrocarbons within the sample to be removed. For example, where theacid to be monitored is thought to contain about percent by weightvolatile and high molecular weight hydrocarbons, the percentage ofremoval established by the acid pumps is set at a much higher level, say30 percent. Accordingly, 25 percent of the stripped acid would also beremoved via overflow tube 51.

To establish optimum pumping rates for the acid pumps 33 and 60, severaloperating parameters of the gravity-indicating apparatus of the presentinvention must be taken into consideration, among which are: thecapacity of the gravity-indicating apparatus, the size of the samplestream, the maximum time lag permitted to measure the sample, as well asthe weight percent of hydrocarbons within the sample. For a maximum timelag of 12 minutes, an input flow rate of 1.5 gallons per hour, a totalapparatus capacity of about one-half gallon, a second stage pumping rateof about 1.0 gallons per hour provides removal of about 30 percent byweight of the input sample. Accordingly, as the weight percent ofvolatile and high molecular weight hydrocarbons within the sampleincreases, the differential flow rate of the acid pumps can be readilyadjusted to facilitate their removal.

Removal of the hydrocarbons from the sample in the aforementioned mannerhas been found to provide stripped samples that are not onlysubstantially free of hydrocarbons, but the hydrocarbons that remain, ifany, are substantially constant. Accordingly, the gravity-indicatingapparatus of the present invention has been found to provide resultsthat can be repeated within very small tolerances.

The invention has been described above in conjunction with a specifictype of alkylation process for which it was particularly designed.However, it is useful in conjunction with other forms of sulfuric acidalkylation process and also with other processes wherein strong sulfuricacid is contacted under pressure with normally gaseous hydrocarbons. Insuch processes it is generally desirable to withdraw a sample of theacid from time to time for control purposes. The invention provides aready means of obtaining such sample without the customary difficultiesarising from the sudden volatilization of the hydrocarbons in thewithdrawn sample.

I claim:

1. In an apparatus for continuously determining the concentration ofsulfuric acid'in an aliquot continuously sampled from a pressurizedalkylation system line and said aliquot having hydrocarbons therein, inwhich said determination occurs after said hydrocarbons have beenremoved from said acid, the improvement comprising, in combination, asampling line connected to said pressurized alkylation system line forcontinuously removing a sample of said acid from said system line,chamber means including overflow tube means, input pumping means havinga suction side connected to said sampling line and a discharge sideconnected to said chamber means and operating to flow said sample tosaid chamber means at a substantially constant flow rate, output pumpingmeans connected to the discharge side of said chamber means for flowingsaid sample from said chamber at an output pumping rate less than thatof said input pumping means, said pumping rates of said input and outputpumping means adapted to define a differential pumping ratetherebetween, the ratio of said differential pumping rate over the inputpumping rate being at least as large as the weight fraction of saidhydrocarbons to be removed from said original acid sample, and means insaid chamber means for measuring a specific gravity characteristic ofsaid sample at a constant temperature after removal of said hydrocarbonsso as to indicate the concentration of said acid.

2. Apparatus in accordance with claim 1 in which said chamber meansincludes a stripping chamber, a settling chamber in direct fluidcommunication with said stripping chamber, and heating means in contactwith said stripping and settling chambers, said stripping chamberincluding vent means, wall means and substantially horizontal extendingplanar bottom base means, said side wall and bottom base means adaptedto convey said input sample of acid flowing from said discharge side ofsaid first input pumping means in open channel heat transfer contactwith said heating means so as to remove volatile hydrocarbons withinsaid acid sample.

3. Apparatus in accordance with claim 2 in which said stripping chamberincludes baffling means connected to said wall and bottom base means andadapted to cause said acid sample to be conveyed in serpentine fashionthrough said stripping chamber so as to improve heat transfer betweensaid heating means and said sample.

4. Apparatus in accordance with claim 2 in which said settling chamberincludes wall means defining a settling cavity therein for receivingsaid sample from said stripping chamber and an output means connected tosaid output fluid pumping means at a remote end thereof for flowing saidsample from said chamber means under pressure after said sample haspassed through said stripping chamber and said settling chamber,

overflow tube means adapted to be positioned within said settling cavityso as to define a skimming level therein for removing high molecularweight hydrocarbons from said sample,

said means for measuring said specific gravity characteristic also beingpositioned in said settling cavity below said skimming level.

5. Apparatus in accordance with claim 4 in which said overflow tubemeans is positioned to establish said skimming level at a datum planeabove said bottom base means of said stripping chamber therebyregulating the depth of said sample of acid in said stripping chamber inopen channel heat transfer contact with said heating means.

6. The method of determining the concentration of sulfuric acid catalystin an alkylation process system which comprises:

1. continuously flowing at a selected pumping rate a sample of said acidto an analysis chamber,

2. removing volatile hydrocarbons from said sample by heating saidsample within said chamber,

3. after removal of said volatile hydrocarbons, removing high molecularweight hydrocarbons from said sample by establishing a predeterminedskimming level within said chamber, said high molecular weighthydrocarbons being removed from said chamber by means of an overlow tubetherein,

4. after removal of said volatile and high molecular weighthydrocarbons, flowing at a second pumping rate said acid sample fromsaid chamber, the pumping rate of said flow from said chamber adapted tobe less than that into said chamber to establish a differential pumpingrate therebetween, the ratio of said difierential pumping rate over theinput flow rate being at least as large as the total weight fraction ofsaid volatile and high molecular weight hydrocarbons to be removed fromsaid sample,

3,653,835 9 l9 5. after removal of said hydrocarbons but prior toflowing 7. Method of claim 6 wherein said differential pumping rate saidsample from said chamber, measuring a specific is adapted to be adjustedso that said ratio exceeds the exgravity characteristic of said acidsample at a known tem- Pected weigh} fraction of both "Q and said highperature to provide a continuous indication of the conmolecular weghthydrocarbons wlthl" 531d p centrationofsaid acid sample. w awi:

2. Apparatus in accordance with claim 1 in which said chamber meansincludes a stripping chamber, a settling chamber in direct fluidcommunication with said stripping chamber, and heating means in contactwith said stripping and settling chambers, said stripping chamberincluding vent means, wall means and substantially horizontal extendingplanar bottom base means, said side wall and bottom base means adaptedto convey said input sample of acid flowing from said discharge side ofsaid first input pumping means in open channel heat transfer contactwith said heating means so as to remove volatile hydrocarbons withinsaid acid sample.
 2. removing volatile hydrocarbons from said sample byheating said sample within said chamber,
 3. after removal of saidvolatile hydrocarbons, removing high molecular weight hydrocarbons fromsaid sample by establishing a predetermined skimming level within saidchamber, said high molecular weight hydrocarbons being removed from saidchamber by means of an overlow tube therein,
 3. Apparatus in accordancewith claim 2 in which said stripping chamber includes baffling meansconnected to said wall and bottom base means and adapted to cause saidacid sample to be conveyed in serpentine fashion through said strippingchamber so as to improve heat transfer between said heating means andsaid sample.
 4. Apparatus in accordance with claim 2 in which saidsettling chamber includes wall means defining a settling cavity thereinfor receiving said sample from said stripping chamber and an outputmeans connected to said output fluid pumping means at a remote endthereof for flowing said sample from said chamber means under pressureafter said sample has passed through said stripping chamber and saidsettling chamber, overflow tube means adapted to be positioned withinsaid settling cavity so as to define a skimming level therein forremoving high molecular weight hydrocarbons from said sample, said meansfor measuring said specific gravity characteristic also being positionedin said settling cavity below said skimming level.
 4. after removal ofsaid volatile and high molecular weight hydrocarbons, flowing at asecond pumping rate said acid sample from said chamber, the pumping rateof said flow from said chamber adapted to be less than that into saidchamber to establish a differential pumping rate therebetween, the ratioof said differential pumping rate over the input flow rate being atleast as large as the total weight fraction of said volatile and highmolecular weight hydrocarbons to be removed from said sample,
 5. afterremoval of said hydrocarbons but prior to flowing said sample from saidchamber, measuring a specific gravity characteristic of said acid sampleat a known temperature to provide a continuous indication of theconcentration of said acid sample.
 5. Apparatus in accordance with claim4 in which said overflow tube means is positioned to establish saidskimming level at a datum plane above said bottom base means of saidstripping chamber thereby regulating the depth of said sample of acid insaid stripping chamber in open channel heat transfer contact with saidheating means.
 6. The method of determining the concentration ofsulfuric acid catalyst in an alkylation process system which comprises:7. Method of claim 6 wherein said differential pumping rate is adaptedto be adjusted so that said ratio exceeds the expected weight fractionof both said volatile and said high molecular weight hydrocarbons withinsaid sample.